> “colour” is itself a mutation, the French word being «couleur»
That's what the French word is now. But the French word used to be color, which is where the American spelling comes from, a late, backward-looking reform. Are you sure it wasn't colour in French when the English spelling standardized?
I wouldn't bet on that. The u in colour is never pronounced, but neither is the o.[1] The spelling color is much more likely to be motivated by that being the Latin spelling of the word.
[1] I watched Star Trek: The Original Series recently and the cast's pronunciation of "sensors" with the FORCE vowel in the second syllable is really jarring. Do we know if that was an affectation or a natural pronunciation?
Anyway, if someone can't hear the difference in that sample, load it in to an audio editor, and see that (in a time / amplitude graph) the two words have different shapes.
I've not got a tool for graphing the frequency content to hand, so can't easily see how those would be distinguished.
As an American, I pronounce buoy as a two- syllable word: boo-ee.
I have only heard buoy pronounced as “boy” in the northeast, not at all universal across the country.
“Centre” indeed comes directly from French, but “colour” is itself a mutation, the French word being «couleur»